This invention relates to polyfunctional silicon compounds and to binding such compounds to substrates for uses such as a separation media. Conventional C18 (ODS) silica columns are widely accepted as general-purpose stationary phases. However, some drawbacks impede the use of these columns for certain applications, including the peak tailing of basic analytes and “phase collapse” (or “de-wetting”) in a highly aqueous environment. Recent advances in silica synthesis and bonding technology provide solutions to minimize the base tailing using high-density bonding followed by exhaustive end-capping of high purity silica. Densely bonded and highly hydrophobic ODS columns cannot be used in 100% water, since the “phase collapse” usually occurs, causing greatly reduced or irreproducible retention.
Polar-embedded phases were introduced in order to improve the peak shape of basic analytes and to make RP columns fully operational in highly aqueous environment. These phases are primarily hydrophobic but have hydrophilic groups incorporated near the silica surface. The commonly used polar groups are amide, urea, ether and carbamate functionalities. In general, the polar-embedded phases have the following benefits in comparison to conventional C18 packings: they provide good peak shapes of basic analytes, good compatibility with highly aqueous mobile phases, and selectivities that differ from those exhibited by general purpose C18 columns. On the other hand, the polar-embedded phases have their own drawbacks such as significantly decreased retention of basic and non-polar compounds and inferior hydrolytic stability, as compared to conventional C18 columns. Therefore, they are often complementary to C18 columns and operated in a narrower pH range.
Several processes have been developed to produce the hydrolytically stable silica based stationary phases. R. P. Fisk et al. (WO 00/45951) discloses a process for preparing the porous inorganic/organic hybrid silica particles as base solid support for further modifications. After reacting with the silylating agents, such as dimethyl octadecyl chlorosilane, the packing material is stated to demonstrate an enhanced hydrolytic stability within the 1–12 pH range. Another method for making stable silica packings for the HPLC applications, was developed by J. L. Glajch et al. (U.S. Pat. No. 4,705,725). It describes the stable support structures covalently modified by a mono-functional silane, containing two sterically hindered groups bound to a silicon atom. The columns packed with these materials are stated to show enhanced hydrolytic stability at low pHs. However, the use of bulky silylating agents could be disadvantageous, since bonded phases often have lower surface coverage, which may result in the decreased phase stability at elevated pHs. J. J. Kirkland et al. reported the preparation of bidentate silane stationary phases for reversed-phase HPLC (J. J. Kirkland; J. B. Adams, Jr.; M. A. van Straten; H. A. Claessens, Analytic Chemistry, 70: 4344–4352 (1998)). This packing material is stated to provide good hydrolytic stability within a broad range of pH levels (1.5–11.5) and to results in satisfactory column efficiency. G. MaGall (U.S. Pat. No. 6,262,216 B1) described the synthesis and use of polyfunctional silanes with tertiary amine groups containing one derivatizable functionality such as hydroxyl, amino, carboxyl, thio, halo and sulfonate, and two reactive silyl moieties.
An objective of this invention is to provide polyfunctional silylating agents with built-in polar fragments, which provide bonded silica phases with the benefits of the polar-embedded packings, and enhance their longevity. There is a need to provide improved polyfunctional silica compounds which can be used for HPLC stationary phase development.